Silver-containing direct thermographic and photothermographic imaging materials (that is, thermally developable imaging materials) that are imaged and/or developed using heat and without liquid processing have been known in the art for many years.
Silver-containing direct thermographic imaging materials are non-photosensitive materials that are used in a recording process wherein images are generated by the direct application of thermal energy and in the absence of a processing solvent. These materials generally comprise a support having disposed thereon (a) a relatively or completely non-photosensitive source of reducible silver ions, (b) a reducing composition (acting as a black-and-white silver developer) for the reducible silver ions, and (c) a suitable binder. Thermographic materials are sometimes called “direct thermal” materials in the art because they are directly imaged by a source of thermal energy without any transfer of the image or image-forming materials to another element (such as in thermal dye transfer).
In a typical thermographic construction, the image-forming thermographic layers comprise non-photosensitive reducible silver salts of long chain fatty acids. A preferred non-photosensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms, such as behenic acid or mixtures of acids of similar molecular weight. At elevated temperatures, the silver of the silver carboxylate is reduced by a reducing agent for silver ion (also known as a developer), whereby elemental silver is formed. Preferred reducing agents include methyl gallate, hydroquinone, substituted-hydroquinones, hindered phenols, catechols, pyrogallol, ascorbic acid, and ascorbic acid derivatives.
Some thermographic constructions are imaged by contacting them with the thermal head of a thermographic recording apparatus such as a thermal print-head of a thermal printer or thermal facsimile. In such constructions, an anti-stick layer is coated on top of the imaging layer to prevent sticking of the thermographic construction to the thermal head of the apparatus utilized. The resulting thermographic construction is then heated imagewise to an elevated temperature, typically in the range of from about 60 to about 225° C., resulting in the formation of a black-and-white image of silver.
Silver-containing photothermographic imaging materials (that is, photosensitive thermally developable imaging materials) that are imaged with actinic radiation and then developed using heat and without liquid processing, have been known in the art for many years. Such materials are used in a recording process wherein an image is formed by imagewise exposure of the photothermographic material to specific electromagnetic radiation (for example, X-radiation, or ultraviolet, visible, or infrared radiation) and developed by the use of thermal energy. These materials, also known as “dry silver” materials, generally comprise a support having coated thereon: (a) a photocatalyst (that is, a photosensitive compound such as silver halide) that upon such exposure provides a latent image in exposed grains that are capable of acting as a catalyst for the subsequent formation of a silver image in a development step, (b) a relatively or completely non-photosensitive source of reducible silver ions, (c) a reducing composition (acting as a developer) for the reducible silver ions, and (d) a binder. The latent image is then developed by application of thermal energy.
In photothermographic materials, exposure of the photosensitive silver halide to light produces small clusters containing silver atoms (Ag0)n. The imagewise distribution of these clusters, known in the art as a latent image, is generally not visible by ordinary means. Thus, the photosensitive material must be further developed to produce a visible image. This is accomplished by the reduction of silver ions that are in catalytic proximity to silver halide grains bearing the silver-containing clusters of the latent image. This produces a black-and-white image. The non-photosensitive silver source is catalytically reduced to form the visible black-and-white negative image of silver while much of the silver halide, generally, remains as silver halide and is not reduced.
In photothermographic materials, the reducing agent for the reducible silver ions, often referred to as a “developer”, may be any compound that, in the presence of the latent image, can reduce silver ion to metallic silver and is preferably of relatively low activity until it is heated to a temperature sufficient to cause the reaction. A wide variety of classes of compounds have been disclosed in the literature that function as reducing agents for photothermographic materials. Upon heating, and at elevated temperatures, the reducible silver ions are reduced by the reducing agent. This reaction occurs preferentially in the regions surrounding the latent image and produces a negative image of metallic silver having a color that ranges from yellow to deep black depending upon the presence of toning agents and other components in the photothermographic imaging layer(s).
Differences Between Photothermography and Photography:
The imaging arts have long recognized that the field of photothermography is clearly distinct from that of photography. Photothermographic materials differ significantly from conventional silver halide photographic materials that require processing with aqueous processing solutions.
In photothermographic imaging materials, a visible image is created in the absence of a processing solvent by heat as a result of the reaction of a reducing agent incorporated within the material. Heating at 50° C. or more is essential for this dry development. In contrast, conventional photographic imaging materials require processing in aqueous processing baths at more moderate temperatures (from 30° C., to 50° C.) to provide a visible image.
In photothermographic materials, only a small amount of silver halide is used to capture light and a non-photosensitive source of reducible silver ions (for example, a silver carboxylate or a silver benzotriazole) is used to generate the visible image using thermal development. Thus, the imaged photosensitive silver halide serves as a catalyst for the physical development process involving the non-photosensitive source of reducible silver ions and the incorporated reducing agent. In contrast, conventional wet-processed, black-and-white photographic materials use only one form of silver (that is, silver halide) that, upon chemical development, is itself at least partially converted into the silver image, or that upon physical development requires addition of an external silver source (or other reducible metal ions that form black images upon reduction to the corresponding metal). Thus, photothermographic materials require an amount of silver halide per unit area that is only a fraction of that used in conventional wet-processed photographic materials.
In photothermographic materials, all of the “chemistry” for imaging is incorporated within the material itself. For example, such materials include a reducing agent (that is, a developer for the reducible silver ions) while conventional photographic materials usually do not. The incorporation of the reducing agent into photothermographic materials can lead to increased formation of various types of “fog” or other undesirable sensitometric side effects. Therefore, much effort has gone into the preparation and manufacture of photothermographic materials to minimize these problems.
Moreover, in photothermographic materials, the unexposed silver halide generally remains intact after development and the material must be stabilized against further imaging and development. In contrast, silver halide is removed from conventional photographic materials after solution development to prevent further imaging (that is in the aqueous fixing step).
Because photothermographic materials require dry thermal processing, they present distinctly different problems and require different materials in manufacture and use, compared to conventional, wet-processed silver halide photographic materials. Additives that have one effect in conventional silver halide photographic materials may behave quite differently when incorporated in photothermographic materials where the underlying chemistry is significantly more complex. The incorporation of such additives as, for example, stabilizers, antifoggants, speed enhancers, supersensitizers, and spectral and chemical sensitizers in conventional photographic materials is not predictive of whether such additives will prove beneficial or detrimental in photothermographic materials. For example, it is not uncommon for a photographic antifoggant useful in conventional photographic materials to cause various types of fog when incorporated into photothermographic materials, or for supersensitizers that are effective in photographic materials to be inactive in photothermographic materials.
These and other distinctions between photothermographic and photographic materials are described in Unconventional Imaging Processes, E. Brinckman et al. (Eds.), The Focal Press, London and New York, 1978, pp. 74-75, in D. H. Klosterboer, Imaging Processes and Materials, (Neblette's Eighth Edition), J. Sturge, V. Walworth, and A. Shepp, Eds., Van Nostrand-Reinhold, New York, 1989, Chapter 9, pp. 279-291, in C. Zou et al., J. Imaging Sci. Technol. 1996, 40, pp. 94-103, and in M. R. V. Sahyun, J. Imaging Sci. Technol. 1998, 42, 23.
Problem to be Solved:
Photothermographic materials are commercially available for use in the medical imaging industry, and are particularly used for diagnosis and archival of clinical images. These materials are currently widely used in regions of the world where viewing and storage of imaged films is done in a controlled environment and at moderate temperature and humidity. However, photothermographic materials are now also being used in regions where the environment for viewing and storage of imaged films is less controlled and the imaged films may be stored at higher temperatures and humidity.
A challenge in photothermographic materials is the need to improve their stability at ambient temperature and relative humidity during storage prior to imaging. This stability is referred to as “Raw Stock Keeping” (RSK), “Natural Age Keeping” (NAK), “Shelf-Life Stability”. It is desirable that photothermographic materials be capable of maintaining their imaging properties, including photospeed and Dmax, while minimizing any increase in Dmin during storage periods. Raw Stock Keeping is a problem especially for photothermographic films compared to conventional silver halide photographic films because, as noted above, all the components needed for development and image formation in photothermographic systems are incorporated into the imaging element, in intimate proximity, prior to development. The large number of potentially reactive components that can prematurely react during storage that can lead to loss of photospeed, increase in Dmin, and changes in other sensitometric properties during storage. These problems can become more acute when photothermographic materials are stored prior to use in severe environmental conditions such as high humidity or high temperature.
One particular type of Raw Stock Keeping problem is “Shelf-aging Fog” or “Dmin aging”. Shelf-aging fog is the increase in Dmin in non-imaged areas of photothermographic materials. The stored, non-imaged materials, upon later imaging and development have a higher Dmin in non-imaged areas when compared to freshly prepared samples of the same materials that have been imaged soon after coating. Shelf-aging fog shortens the shelf-life of photothermographic materials. A great amount of work has been done to improve the shelf-life characteristics of photothermographic materials. In order to destroy fog centers or to limit their growth during development, additional additives, such as stabilizers and antifoggants, have been incorporated into the imaging layers.
Yet another problem encountered in photothermography is that the photospeed and other sensitometric properties often change as the material ages during storage.
U.S. Pat. No. 4,161,408 (Winslow et al.) describes the preparation of silver halide/silver carboxylate soaps by converting a melted long chain fatty acid in water to an ammonium or a mixture alkali metal salts. Conversion by adding a mixture of sodium and lithium hydroxides is described. The ammonium or alkali metal salts are then converted to the silver salt of the acid.
U.S. Pat. No. 7,175,978 (Maeda et al.) describes the preparation of silver halide/silver carboxylate soaps by converting a melted long chain fatty acid in water to a mixture alkali metal salts. Conversion by adding a mixture of sodium and potassium hydroxides is described. The alkali metal salts are then converted to the silver salt of the acid.
There remains a need for methods to provide photothermographic materials whose photospeed and other sensitometric characteristics undergo little change as the material ages during, storage. That is, photothermographic materials with improved shelf stability are needed.